As is known per se, an aircraft engine nacelle comprises at least one cowl able to move between a service position, in which this cowl covers the engine, and a maintenance position, in which this cowl is distanced from the engine and thus allows the technician to work on the engine or on the underside of the cowl, to carry out maintenance operation.
Given the high weight of such a cowl, particularly on large nacelles such as those used on the Airbus A380, it is absolutely essential that means for assisting the opening of this cowl be provided.
These assistance means, often known even in other languages by their English-language abbreviation PCOS (which stands for “Powered Cowl Operating System”), generally comprise at least one hydraulic or electric actuator which can be made to open or to close the cowl.
As is known per se, an actuator such as this comprises, at its end collaborating with the cowl, a device often known, even in other languages, by its English name as “free play” device, which always allows the end of this actuator a small additional travel for a given extension of the actuator.
The purpose of this free play is, on the one hand, to avoid compression/tension forces being transmitted to the actuator in a flight situation as the cowl experiences deformation associated with particular pressure profiles and, on the other hand, to allow easy manual closure of the cowl at the end of the maintenance operations in spite of the friction generated by the tolerances on the various components which interact.
A jury strut or prop is also provided so as to block the cowl in the open position and relieve the actuator of the weight of this cowl. This jury strut is fitted by hand, by the technician, and removed by the same just before the cowl is closed.
In practice, just after having opened the cowl using the actuator and after having fitted the jury strut, the technician causes the actuator to retract slightly so as to make sure that the weight of the cowl is indeed being borne by the jury strut, rather than by the actuator: when this is done, the free play device extends at least partially.
Now, the jury strut may sometimes break under the weight of the cowl and/or of the loads applied by the actuator instructed to close by the operator; because of the presence of the free play device which is initially in an at least partially extended position, there will be a veritable dropping of the cowl onto the actuator, from a height that corresponds to the extension of the free play device.
This drop comes as a great shock to the technician working between the cowl and the engine, and may even injure him. In addition, this sudden sharp transfer of the weight of the cowl onto the actuator may damage this actuator because of the additional dynamic effect introduced.